A three-year federal corruption investigation of pay-to-play politics in Gov. Rod Blagojevich’s administration has expanded to include his impending selection of a new U.S. senator to succeed President-elect Barack Obama, the Tribune has learned.

Federal authorities got approval from a judge before the November general election to secretly record the governor, sources told the Tribune, and among their concerns was whether the selection process might be tainted. That possibility has become a focus in an intensifying investigation that has included recordings of the governor and the cooperation of one of his closest friends.